An image recording apparatus of the type using a laser beam as an image forming means is known and in use. In an image recording apparatus of this type, an image is to be formed on the surface of a light-sensitive sheet medium transported to a predetermined elongated or linear light incidence zone at which the sheet medium is to be irradiated with a laser beam emitted from a laser beam generator located in conjunction with the light incidence zone. Lines of images are thus formed successively on the light-sensitive sheet medium as the sheet medium is passed through the light incidence zone. The sheet medium is driven to travel through the light incidence zone synchronously as lines of images are to be produced one after another on the light-sensitive sheet medium.
In order to enable a light-sensitive sheet medium to be driven to move in synchronism with the irradiation with the laser beam, a known laser-beam image recording apparatus ordinarily has means to detect an arrival of the leading edge of the sheet medium at a predetermined location with respect to the light incidence zone at which the sheet medium is to be irradiated with a laser beam Such detecting means is implemented typically by a mechanically activated switch such as a microswitch or a lead switch located in the path of the light-sensitive sheet medium to be transported toward the light incidence zone of the apparatus. The microswitch or lead switch thus located with respect to the light-sensitive sheet being transported toward the light incidence zone detects an arrival of the leading edge of the sheet medium at a predetermined location with respect to the light incidence zone. In a predetermined period of time after the leading edge of the sheet medium was initially detected by the switch, irradiation of the sheet medium with a laser beam to form a line of images is started on the assumption that the leading edge of the sheet medium has reached the light incidence zone of the apparatus.
A problem is however encountered in a prior-art laser-beam image recording apparatus of this type in that the timing at which the leading edge of a light-sensitive sheet medium is to be detected may subtly vary from one sheet medium to another since such a timing is dictated solely by the mechanically activated switch means. Another important problem of the known laser-beam image recording apparatus is the inability of the switch means to detect the leading edge of a light-sensitive sheet medium because of the arrangement in which the switch means is located at a considerable distance from the light incidence zone of the apparatus.
By reason of these problems, it may happen in a prior-art laser-beam image recording apparatus of the described type that the apparatus fails to start the irradiation of a light-sensitive sheet medium with a laser beam to form a line of images although the leading edge of the sheet medium has already reached the light incidence zone of the apparatus. When this occurs, there will be produced on a leading end portion of the sheet medium an area which has failed to be exposed to a laser beam and which accordingly is to be undesiredly left blank. Where images are to be formed throughout the longitudinal measurement of the light-sensitive sheet medium, it may also happen that the lines of images which should have been formed on a trailing end portion of the sheet medium may fail to be formed thereon. If, on the contrary, the irradiation of the light-sensitive sheet medium with a laser beam is started before the leading edge of the light incidence zone of the apparatus, then it may happen that lines of images which should have been formed on a leading end portion of the sheet medium may fail to be formed thereon or there will be produced on a trailing end portion of the sheet medium an area which has failed to be exposed to the laser beam and which is to be undesiredly left blank.
These problems may be alleviated or even practically eliminated if improvements are made so that a light-sensitive sheet medium is to be driven to travel at a more precisely controlled velocity and the sheet medium is to be irradiated at more precisely controlled timings. If such improvements are made successfully, however, still another problem is encountered in a prior-art laser-beam image recording apparatus of the described type, which results from the fact that the light-sensitive sheet media to be used, which are ordinarily provided in the form of standard-sized sheets of paper, are subject to fluctuations in size. Where a roll of paper consisting of a continuous strip of paper is used and is cut into sheets of a desired longitudinal measurement, the resultant sheets of paper will be subject fluctuations in the longitudinal measurement thereof due to the insufficient accuracy in cutting the rolled sheet. If a sheet medium has a length less than the desired longitudinal measurement, some lines of images may fail to be formed on the sheet medium and if, conversely, than the sheet medium has a length greater than the desired longitudinal measurement, the sheet medium may have an unexposed trailing end. Essentially similar problems will arise if the light-sensitive sheet media to be used have lateral measurements which fluctuate from one sheet medium to another. Missing of some lines of images or formation of an unwanted blank area may also be caused if the light-sensitive sheet medium passed to the light incidence zone of the apparatus skews through the zone.